1. Field of the Invention
The invention relates to a single mode optical fibre which may be used for transmitting radiation at substantially higher powers than may be achieved using conventional means. The fibre does not suffer from non linear effects or optical damage at high powers to the same extent as conventional optical fibres. In particular, the fibre may be used as a single mode waveguide, in a single mode fibre laser or in a single mode fibre amplifier.
2. Discussion of Prior Art
Optical fibres are widely used to deliver light from one point to another and have application in communications, imaging and sensing. Conventionally, a typical optical fibre is a long strand of transparent material which is uniform along its length but which has a refractive index varying across its cross-section. For example, a central core region of higher refractive index is surrounded by a cladding region with a lower refractive index. Such a fibre may be made from fused silica with a cladding of pure silica surrounding a core made from silica into which deliberate impurities have been introduced to raise the refractive index. Light is confined in or near the core by the process of total internal reflection which takes place at the boundary between the core and the cladding.
In general, a fibre of this type may support more than one guided mode of propagation confined to the core (i.e. multi mode fibre), these modes travelling along the fibre at different phase velocities. However, if the core is made to be sufficiently small, only one guided mode of propagation will be confined to the core, the fundamental mode (i.e. a single mode fibre). That is, the distribution of light emerging from the fibre is unchanged when the conditions at the launch end of the fibre are chanced and when the fibre itself is subjected to disturbances such as transverse compression or bending. Typically, a fibre designed to carry single mode light having a wavelength of 1500 nm may have a few percent of germanium dopant in the core, with a core diameter of 9 xcexcm.
More recently, a photonic crystal fibre (PCF) has been developed comprising a cladding made of a transparent material in which an array of holes are embedded along the length of the fibre [J. C. Knight, et al., Opt. Lett. 21 (1996) p. 1547. Errata: Opt. Lett. 22 (1997) p. 484]. The holes are arranged transversely in a periodic array and are filled with a material which has a lower refractive index than the rest of the cladding, the core of the fibre comprising a transparent region which breaks the periodicity of the cladding. Typically, both the core and the cladding are made from pure fused silica and the holes are filled with air. The core diameter is approximately 5 xcexcm and the flat-to-flat width of the whole fibre is around 40 xcexcm. Pith a hole spacing of around 2-3 xcexcm. If the diameter of the air holes in the fibre is a sufficiently small fraction of the pitch or spacing between the holes, the core of the fibre guides light in a single mode.
Single mode fibres have advantages over multi mode fibres in the field of long distance telecommunication, laser power delivery and many sensor applications due to the fact that a light signal carried by the fibre travels in only one mode and therefore avoids the problem of intermodal dispersion that is encountered with multi mode fibres. Also, at a given wavelength the intensity of light across a single mode fibre is guaranteed to follow a single smooth, known and unchanging distribution. This is regardless of how light is launched into the fibre or of any disturbance of the fibre (e.g. time varying).
In many applications it is advantageous for an optical fibre to carry as much optical power as possible as, for example, any fibre inevitably attenuates the light passing through it. For example, for a given detector sensitivity, the length of a communications link can be increased by increasing the radiation power input to the fibre. As another example, there are many high power laser systems in industrial applications which could be made more simply if light could be channelled via a fibre rather than using conventional optical systems. There are, however, limits to the amount of light that can be carried by known optical fibres at a given time.
In a conventional fibre, comprising a core region surrounded by a lower refractive index cladding region, the material from which the fibre is made will ultimately suffer irreversible damage if the light intensity within the fibre exceeds a threshold value. At lower intensities, a number of intensity dependent non-linear optical processes can occur which, although non destructive to the fibre, nevertheless can degrade or even destroy an optical signal.
These problems may be alleviated by increasing the size of the core of the fibre which, for a given power, reduces the intensity of the light in the fibre, therefore allowing a greater power to be carried before the threshold for non linear processes are reached. However, if the core diameter alone is increased the fibre will become multi mode. This may be compensated by reducing the index difference between the core and the cladding. Eventually, however, it becomes difficult to control the uniformity of doping across the core. Furthermore, fibres with small index differences are susceptible to loss of light at bends. Therefore, there are limits to the extent to which an increased core size can be used to increase the power capacity of a single mode fibre.
Some of the non-linear effects are exacerbated by the presence of dopants in the core, which make the material more susceptible to these effects. At higher powers, doped fibres are more susceptible to irreversible damage. Dopants also make the fibre more susceptible to damage by ionising radiation which is an issue in the nuclear industry. This has been combated by making the core out of pure silica. Total internal reflection is maintained by introducing dopants to the cladding which reduce its refractive index and as less light is carried in the cladding than in the core, more power can be carried. However, this is limited by the fact that some of the light is carried in the doped cladding.
Furthermore, in conventional fibres, efficient coupling of high power lasers into the fibre is problematic as the light needs to be focused into a small spot and the intensity at the endface of the fibre is therefore larger than it would be if the core were larger. Optical damage at or near the endface of the fibre frequently limits the power of radiation that can be launched into it [S. W. Allison et al., Appl. Opt. 24 (1985) p. 3140]. The maximum continuous wave (cw) power that has been achieved in a conventional single-mode fibre is only around 15 W.
The invention overcomes the problem of the incompatibility of transmitting high power radiation using conventional fibres whilst retaining single mode behaviour. In particular, the fibre may be used as a waveguide for delivering radiation from one point to another, or may be used in a fibre amplifier or a fibre laser. The fibre may be capable of supporting a single mode of propagation of radiation having a maximum power in the region of 100 W-2 kW. Furthermore, if the core is undoped, the fibre is less susceptible to irreversible damage at high intensities compared to conventional (doped) fibres. The effects of non-linear optical processes in the fibre are reduced and a high power signal output from the fibre does not therefore suffer degradation. The fibre has a further advantage in that high power radiation may be efficiently coupled into the fibre without the need for focusing to a small beam spot size.
According to one aspect of the present invention, an optical fibre for transmitting radiation comprises;
a core comprising a substantially transparent core material having a core refractive index, n, and a length, l, and having a core diameter of at least 5 xcexcm and
a cladding region surrounding the length of core material, wherein the cladding region, comprises a first substantially transparent cladding material, having a first refractive index, and wherein the first substantially transparent cladding material has embedded along its length a substantially periodic array of holes, having a diameter, d, and being spaced apart by a pitch xcex9, wherein the holes have a second refractive index which is less than the first refractive index,
such that the dimensions of the hole diameter d, and the pitch, xcex9, co-operate to give single mode propagation within the optical fibre independent of input radiation wavelength for any value of the pitch, xcex9, for a substantially fixed d/xcex9 ratio.
If the holes have a diameter, d, and are spaced apart by a pitch xcex9, the optical fibre may be single mode independently of input radiation wavelength for any value of the pitch, xcex9, for a substantially fixed d/xcex9 ratio. The invention provides the advantage that the fibre may be made single mode for any wavelength across an extended wavelength range compared to that which may be achieved using conventional fibre. This is because for any wavelength across the extended range, the fibre remains single mode for a fixed d/xcex9 ratio.
Preferably, the first substantially transparent cladding material may have a refractive index not less than the core refractive index. In a preferred embodiment, the core diameter may be at least 10 xcexcm. In a further preferred embodiment, the diameter of the core may be at least 20 xcexcm.
In one embodiment of the invention, at least one hole in the array may be absent such that it forms the core of the optical fibre. The holes may be arranged in a substantially hexagonal pattern.
Preferably, the first substantially transparent cladding material may have a refractive index not less than the core refractive index. In a preferred embodiment, the core diameter may be at least 10 xcexcm. In a further preferred embodiment, the diameter of the core may be at least 20 xcexcm.
In one embodiment of the invention, at least one hole in the array may be absent such that it forms the core of the optical fibre. The holes may be arranged in a substantially hexagonal pattern.
The holes may be a vacuum region or may be filled with a second cladding material. For example, he second cladding material may be any substantially transparent material, may be air or another gas (e.g. hydrogen or hydrocarbon) may be a liquid (e.g. water, any other aqueous solution or a solution of dyes) or may be a solid (e.g. a glass malarial having a different refractive index from that of the first cladding material).
The first substantially transparent cladding material may have a substantially uniform first refractive index and the core material may have a substantially uniform core refractive index. The core material and the first substantially transparent cladding material may be the same material. For example, at least one of the core material and the first substantially transparent cladding material may be silica. Preferably, the diameter of the holes is not less than the wavelength of light to be guided in the fibre. In a preferred embodiment of the invention the spacing between the holes, xcex9, is not less than one quarter of the core diameter, c, and not more than one half of the core diameter, c
In one embodiment of the invention, the substantially transparent core material may comprise a dopant material, for example rare earth ions such as erbium.
According to a second aspect of the invention, a fibre amplifier for amplifying signal radiation comprises;
a length of the optical fibre as described herein, for receiving signal radiation of selected wavelength and transmitting said signal radiation along its length, wherein the core material comprises a dopant material along at least part of its length,
a source of radiation for emitting pump radiation of a different selected wavelength for input to the length of the optical fibre, such that said part of the doped core material amplifies the signal radiation under the action of the pump radiation and
wavelength-selective transmission means for selectively transmitting the pump radiation into the length of the optical fibre and for selectively outputting the amplified signal radiation from the fibre amplifier.
For example, the wavelength-selective transmission means may comprise an input lens and an output lens for focusing radiation and a dichroic mirror for selectively reflecting pump radiation into the optical fibre and for and selectively transmitting the amplified input radiation to be output from the fibre amplifier. Alternatively, the wavelength-selective transmission means comprise a fibre directional coupler having a wavelength dependent response.
The dopant material may comprise rare earth ions, for example erbium ions.
According to another aspect of the invention, a fibre laser for outputting laser radiation comprises;
a length of the optical fibre as herein described for selectively transmitting laser radiation having a selected wavelength along its length, wherein at least part of the length of the core material comprises a dopant material,
a source of radiation for emitting pump radiation of a different selected wavelength for input to the length of the optical fibre, such that said doped core material amplifies the laser radiation under the action of the pump radiation.
wavelength-selective transmission means for selectively transmitting the pump radiation into the length of the optical fibre and for selectively outputting the amplified laser radiation from the fibre laser and
feedback means for selectively feeding back a part of the amplified laser radiation such that said amplified laser radiation passes along the length of the optical fibre repeatedly and is further amplified.
The dopant material may comprise rare earth ions, such as erbium ions.
In one embodiment of the fibre laser, the wavelength-selective transmission means and the feedback means together may comprise two dichroic mirrors, wherein each of the dichroic mirrors are situated at different positions along the length of the optical fibre and wherein the doped core material is situated between the positions of the two dichroic mirrors.
In an alternative embodiment of the fibre laser, the feedback means and the wavelength-selective transmission means together may comprise two fibre gratings formed at two different positions along the length of the optical fibre such that the doped core material is situated between the two fibre gratings.
In another embodiment of this aspect of the invention, the fibre laser may be a ring resonator fibre laser wherein the feedback means comprise:
means for directing light emerging from one end of the length of optical fibre having a doped core material into the other end of said length of optical fibre.
According to another aspect of the invention, a system for transmitting radiation in a single mode of propagation comprises;
a plurality of lengths of the optical fibre as herein described arranged in a series such that each length of optical fibre receives input radiation from the previous length of optical fibre in the series and transmits output radiation to the subsequent length of the optical fibre in the series and each length being separated by amplification means for amplifying the radiation output from a length of the optical fibre so as to maintain the power of the radiation transmitted by the lengths of optical fibre above a predetermined power.
In a preferred embodiment, the amplification means may comprise a fibre amplifier as herein described.